Multicore plastic optical fiber for light signal transmission

ABSTRACT

The present invention relates to a multicore plastic optical fiber for light signal transmission comprising 7 or more cores having a diameter of 50 to 200 μm which are covered with a cladding resin having a refractive index lower than that of the core resin by 0.005 to 0.04. The multicore plastic optical fiber of the present invention has low transmission loss in a broad transmission bandwidth and exhibits excellent bending characteristics. Therefore, it is suitable for transmission at a high speed in short and medium distances.

TECHNICAL FIELD

The present invention relates to a multicore plastic optical fiber forlight signal transmission which has low transmission loss in a broadtransmission bandwidth and does not experience greatly increasedtransmission loss even during bending of the fiber.

BACKGROUND OF THE INVENTION

Heretofore, a plastic optical fiber cable prepared by covering a singlecore plastic optical fiber with a thermoplastic resin such aspolyethylene, by thinly covering a core having a diameter of about 0.5to 1.5 mm with a cladding material in a concentric configuration, isemployed for optical communication. Plastic Optical Fiber andApplications Conference '92, PROC., 3 (1992) reports that conventionalstep index type single core plastic optical fibers have a transmissionbandwidth of 40 MHz when the fiber length is 100 meters and the launchnumerical aperture of the light source is 0.65. However, when the lightsource launch numerical aperture is collimated to 0.01, its transmissionbandwidth becomes 115 MHz in the case of a 100-meter long fiber.

A bundle cable prepared by covering a bundle of some tens of thinplastic optical fibers having a diameter of 0.125 to 0.25 mm with athermoplastic resin has been also applied to practical use. Since thefibers composing the bundle cable are not fused each other, they shiftand make the end of the cable uneven. As a result, the light capacity ofthe cable is considerably reduced. Further, when a bundle cablecomprising plastic fibers having a small diameter is used, the opticalfibers are easily snapped or transmission loss greatly increases whenthe cable is covered with a thermoplastic resin.

Japanese Patent Application Laid-Open No. 265606/1987 discloses a singlecore plastic optical fiber using a methacrylate fluoride type copolymerand the like as a cladding material. This fiber has a relatively smalltransmission bandwidth so that it is only applicable to short distancesignal transmission at a low speed such as audio or factory automation(FA).

On the other hand, Japanese Patent Application Laid-Open No. 53035/1993discloses a multicore plastic fiber for signal transmission comprising500 or more cores which are made of a transparent resin possessing ahigh refractive index, and have a diameter of 50 μm or less. This fiberminimizes changes in light capacity upon bending by making the corediameter 50 μm or less.

Japanese Patent Application Laid-Open No. 341147/1993 discloses anmulticore type single mode optical fiber about 5 meters long whichefficiently links a silica optical fiber with a light source or a lightreceiving element and a transmission method using the fiber. Since thisfiber satisfies the single mode transmission conditions, its corediameter is quite small, 5.3 μm, when the difference of refractiveindexes between the core resin and the cladding resin is 0.003 or more.

Some multicore optical fibers are put into practice as an image fiber,not as a fiber for light signal transmission. An image fiber transmitsan image from one tip of the fiber to the other while maintaining apicture focused at one tip and a positional relation of optical strengthpatterns. Accordingly, the purpose of an image fiber is completelydifferent from that of the light signal transmission fiber.

As described above, the conventional plastic optical fibers do notachieve simultaneously a low transmission loss at a broad transmissionbandwidth and a small change in light capacity upon bending the fiber.In order to improve the conventional plastic optical fibers and developa useful plastic optical fiber which is applicable to a middle-distancesignal transmission at a high speed such as a local area network (LAN),the present inventors carried out extensive and intensive studies andaccomplished the present invention.

DISCLOSURE OF THE INVENTION

The present invention relates to a multicore plastic optical fiber forlight signal transmission comprising 7 or more cores having a diameterof 50 to 200 μm which are covered with a cladding resin having arefractive index lower than that of the core resin by 0.005 to 0.04.

In the present invention, the core diameter is 50 to 200 μm. When thediameter is smaller than 50 μm, transmission loss is increased or noisemay occur in the case that a laser is used for a light source. When thediameter exceeds 200 μm, light loss is considerably increased on bendingthe optical fiber.

To increase the transmission speed, the difference of refractive indexesbetween the core and cladding resins must be minimized. However, whenthe difference is small, the light capacity is remarkably reduced whenbending the fiber. In order to prevent reduction in the light capacity,the core diameter should be made small. Accordingly, the core diameteris determined by transmission speed, transmission distance in use andbending radius when wired. For example, when a 125 to 300 MHz signal istransmitted 100 meters, the preferable core diameter is 130 to 200 μm;when a 200 to 400 MHz signal is transmitted 100 meters, the preferablecore diameter is 80 to 170 μm; and in the case of a high speedtransmission of a signal of more than 400 MHz, the preferable corediameter is 50 to 80 μm. However, the core diameter is not limited tothe above-mentioned ranges.

In order to obtain a uniform fiber cross-section, the number of cores is7 or more which achieves stable core disposition. The preferable corenumber is 19 or more. For making the outer diameter of a fiber 3 mm, thecore number is 4,000 or less.

The cores are preferably arranged in the closest packing structure. Whenthe core number is relatively small, the cores positioned on theoutermost periphery are preferably on the same circumference. Forexample, a preferable core disposition is a 7-core dispositioncomprising one core uniformly surrounded with 6 cores in a circle and a19-core disposition comprising 7 cores arranged in the 7-coredisposition further surrounded with 12 cores in a circle.

As the core diameter becomes smaller, the interface between the core andthe cladding is more easily deteriorated or deformed due to heat andpressure caused during covering of the core, so that transmission lossis likely to increase. The preferable core number to minimize suchinfluence is 19 or more. In the case of a multicore plastic opticalfiber comprising 19 cores, the 7 inner cores and the 12 cores at theoutermost layer are regarded as cores for transmission and as aprotection layer, respectively. Transmission loss is deteriorated onlyin the outermost layer and the transmission loss of the inner 7 cores isnot deteriorated even if damaging conditions have occurred duringcovering of the fiber.

As the core material, there can be used various transparent resins suchas a methyl methacrylate type resin, a styrene type resin, apolycarbonate type resin and an amorphous polyolefin type resin. Ofthese, the methyl methacrylate type resin is preferable since it hashigh transparency so that long distance transmission can be attained.

The methyl methacrylate type resin includes a methyl methacrylatehomopolymer and a copolymer containing methyl methacrylate in an amountof 50% by weight or more. The latter copolymer can be obtained bysuitably selecting and copolymerizing one or more components fromcopolymerizable components such as acrylic esters like methyl acrylate,ethyl acrylate and n-butyl acrylate, methacrylic esters like ethylmethacrylate, propyl methacrylate and cyclohexyl methacrylate,maleimides, acrylic acid, methacrylic acid, maleic anhydride andstyrene.

The styrene type resin includes a styrene homopolymer and a copolymerobtained by copolymerizing styrene, and one or more other componentssuch as an acrylonitrile-styrene copolymer, a styrene-methylmethacrylate copolymer, a styrene-maleic anhydride copolymer and astyrene-6-membered ring acid anhydride. The styrene type resin ispreferred because it is hardly affected by water due to its smallhygroscopicity.

The polycarbonate type resin includes aliphatic polycarbonate andaromatic polycarbonate which are represented by the following formula(1) wherein R is represented by the following formula (2) and formula(3), respectively. ##STR1##

The polycarbonate type resin also includes a copolymer of theabove-mentioned polycarbonates and dioxy compounds such as4,4-dioxyphenyl ether, ethylene glycol, p-xylene glycol and 1,6-hexanediol, a hetero-bond copolymer containing ester bonds in addition tocarbonate bonds, and the like. The polycarbonate type resin is preferreddue to its high heat resistance and small hygroscopicity.

The amorphous polyolefin resin includes resins manufactured in Japansuch as "ARTON" (trade name, manufactured by Japan Synthetic Rubber Co.,Ltd.), "APO" (trade name, manufactured by Mitsui PetrochemicalIndustries, Ltd.) and "ZEONEX" (trade name, manufactured by Nippon ZeonCo., Ltd.). The amorphous polyolefin resin is preferred due to itsexcellent heat resistance.

The refractive index of the core resin is preferably 1.47 to 1.60. Themelt index of the core resin is preferably 1 g/10 min. to 5 g/10 min.though it is not particularly limited if the core resin can be spun.

The refractive index of the cladding resin must be lower than that ofthe core resin by 0.005 to 0.04, preferably 0.01 to 0.04. When thedifference of the refractive indexes is less than 0.005, the amount oflight taken into the plastic optical fiber becomes too small since therefractive index of the cladding resin is too close to that of the coreresin; as the result, the light cannot be detected by a light detector.When the difference of refractive indexes is more than 0.04, asufficient transmission bandwidth cannot be obtained.

When the difference of refractive indexes of core and cladding resins issmall, the light loss becomes large when bending the fiber. In thepresent invention, the light loss upon bending is reduced by making thecore diameter 200 μm or less, and the reduction of the amount of lighttransmitted caused by the smaller core diameter is compensated by themultiple cores.

The melt index of the cladding resin is preferably 5 g/10 min. to 60g/10 min, more preferably 20 g/10 min to 40 g/10 min. When the meltindex is small, it is difficult to mold the resin in the shape of afiber since the diameter is considerably changed by the high fictionalresistance against the die wall. When the melt index is large, it isdifficult to maintain the diameter constant at molding since thecladding resin flows toward the die head faster than the core resin.

The cladding resin is not particularly restricted if it satisfies theabove conditions. The preferable cladding resin includes a resincontaining the above-mentioned core resin components at a slightlydifferent composition ratio from the composition of the above-mentionedcore resin components, a resin containing at least one component of thecore resin components, a resin prepared by copolymerizing or blendingthe core resin component(s) and at least one other components, and thelike. The above-listed resins have a good adhesion property to the coreresin and their other physical properties are similar to the core resin.Consequently, the reliability of the optical fiber comprising suchresins is enhanced.

A cladding resin used for a fiber whose core is made of a methylmethacrylate type resin includes a methacryl type resin and/or anacrylate type resin and/or a fluorinated vinylidene type resin and othertypes of resins. These resins are preferred for minimizing thedifference of refractive indexes of the core and cladding resins. Theresins include a copolymer containing, as a main monomer, a fluorinatedmethacrylate such as trifluoroethyl methacrylate, tetrafluoropropylmethacrylate, pentafluoropropyl methacrylate, heptadecafluorodecylmethacrylate and octafluoropropene methacrylate, a methacrylate typemonomer such as methyl methacrylate, ethyl methacrylate, propylmethacrylate and butyl methacrylate, a fluorinated acrylate such astrifluoroethyl acrylate, tetrafluoropropyl acrylate and octafluoropentylacrylate, and an acrylate type monomer such as methyl acrylate, ethylacrylate, propyl acrylate and butyl acrylate; a fluorinated vinylidenetype copolymer; and a blend of a fluorinated vinylidene type copolymerand a methyl methacrylate type resin. Of these, the resins having asmaller refractive index than a core resin by 0.005 to 0.04 can beemployed for a cladding resin.

Further, if desired, a component such as methacrylic acid,o-methylphenyl maleimide, maleimide, maleic anhydride, styrene, acrylicacid, a hexacyclic compound of methacrylic acid can be added in anamount of 5 parts by weight or less per 100 parts by weight of thecopolymer composition. Specifically, the copolymer composition includesa copolymer of heptadecafluorodecyl methacrylate and methylmethacrylate; a copolymer of tetrafluoropropyl methacrylate and methylmethacrylate; a copolymer of trifluoroethyl methacrylate and methylmethacrylate; a copolymer of pentafluoropropyl methacrylate and methylmethacrylate; a copolymer of heptadecafluorodecyl methacrylate,tetrafluoropropyl methacrylate and methyl methacrylate; a copolymer ofheptadecafluorodecyl methacrylate, trifluoroethyl methacrylate andmethyl methacrylate; a copolymer of heptadecafluorodecyl methacrylate,trifluoroethyl methacrylate, tetrafluoropropyl methacrylate and methylmethacrylate; and the like. The composition ratio of monomers and thelike are determined so as to adjust the refractive index of the claddingresin to be smaller than that of the core resin by 0.005 to 0.04. Ofthese, a copolymer of heptadecafluorodecyl methacrylate, trifluoroethylmethacrylate, tetrafluoropropyl methacrylate and methyl methacrylate ispreferably employed in view of a balance of heat resistance,transparency, mechanical properties and other properties. The abovecombinations do not always necessarily include a fluorine-containingcomponent.

The ratio of the core area to the cladding area in a cross-section ofthe multicore plastic optical fiber of the present invention ispreferably between 9 to 1 and 4 to 6. To increase light transmission,the amount of the cladding resin is preferably minimized within a rangewhere a transmission loss is maintained. When a third encapsulatinglayer is used, the ratio of the core resin to the cladding resin isbetween 9.8 to 0.2 and 9 to 1 and the ratio of the total area of thecore and cladding to the area of the third encapsulating layer ispreferably between 9 to 1 and 4 to 6.

The cross-section of the multicore plastic optical fiber of the presentinvention is usually substantially circular. The diameter of thecross-section is about 0.1 to 0.3 mm, generally about 0.5 to 1.0 mm.

The structure of the cross-section of the present invention can beeither an "islands-in-sea" structure comprising "islands" of many coressurrounded with a "sea" of cladding as shown in FIG. 1 or an"islands-in-sea" structure comprising two-layer "islands" obtained bycovering a core with a cladding which are surrounded with a "sea" of athird encapsulating layer as shown in FIG. 2.

The material of the third encapsulating layer includes polyolefin suchas polyethylene, PVC, a fluorine resin, polyamide, ionomer, anethylene-vinyl acetate copolymer, ABS, polybutylene terephthalate, amethyl methacrylate type resin, a polystyrene type resin, apolycarbonate type resin and elastomers thereof.

The multicore plastic optical fiber of the present invention can beprepared by composite spinning molten core and cladding resins so as tobe an islands-in-sea structure using a special nozzle and two extrudersin clean circumstances substantially free from dust.

Core and cladding resins are charged into a composite spinning die inthe molten state. First, the core resin is supplied to a die plate, onwhich 7 or more holes are substantially uniformly arranged, andsubsequently passed through thin guide tubes. Then, the molten claddingresin is charged around all the thin tubes, in which the core resin isflowing, and spun so as to be a structure comprising the cores asislands in a sea of the cladding resin. In the case that a thirdencapsulating layer is provided, a resin for the third layer is chargedaround the cladding resin. The thus-obtained fiber is stretched so as tobe 1.3 to 3.0 time longer for orientation of molecules and improvementin mechanical properties, to obtain the multicore plastic optical fiberof the present invention.

The outside of the thus-obtained multicore plastic optical fiber iscovered with a resin composition in order to further improve heatresistance and mechanical properties to obtain a multicore plasticoptical fiber cable. As the resin composition for coating, conventionalresin compositions can be used. For example, they include polyethylene,polypropylene, an ethylene-vinyl alcohol copolymer, a rubber, varioustypes of thermoplastic elastomers, polyvinyl chloride, cross-linkedpolyolefin, crosslinked polyvinyl chloride, a chlorinated polyethylenecompound, a polyamide resin, a fluorine resin, a polyester resin, apolyurethane resin, a silicone resin, a thermosetting resin, aultraviolet-curing resin, and mixtures of these resins. Further, thecoating layer may be reinforced by an aramide fiber, a polyacetal fiber,a ultra-high-molecular-weight polyethylene fiber, a metallic fiber andthe like. The thickness of the coating layer is suitably decidedaccording to the circumstances that the cable is actually used. Severalcoating layers can be provided as a multi-layer.

A preferable method for coating a plastic optical fiber with these resincompositions for coating comprises preparing a plastic optical fiber inaccordance with a composite spinning method and coating the fiber with ahot-melt coating material. A plastic optical fiber is coated with amolten resin using a crosshead die, as in electric wire coating.

By preparing the multicore plastic optical fiber of the presentinvention so as to have the above-mentioned structure, the fiber canachieve a low loss of a light capacity at bending in spite of a smalldifference of the refractive indexes between the core resin and thecladding resin. The multicore plastic optical fiber of the presentinvention has a low transmission loss and a broad transmissionbandwidth. It has a transmission bandwidth of 625 MHz·20 meters or morewhen measured using a light source having a launch numerical aperture of0.25. Herein, the transmission bandwidth of 625 MHz·20 meters or moremeans a fiber having a length of 20 meters and a transmission bandwidthof 625 MHz or more. More preferably, the multicore plastic optical fiberof the present invention has a transmission band of 125 MHz·100 metersor more when measured using a light source having a launch numericalaperture of 0.25.

The core filaments of the multicore plastic optical fiber of the presentinvention are fixed in position by a cladding resin. Therefore,differing from a bundle fiber, the relative position of the corefilaments of the multicore plastic optical fiber is the same at anincidence plane and an outgoing plane, and optical power distribution atthe light source is high at the center and low at the peripheral part.On the other hand, a high speed photodiode having a small opticalreceiving diameter of about 0.4 mm requires to gather light strongly atthe center of the photodiode for receiving. Accordingly, the multicoreplastic optical fiber of the present invention is suitable for such aphotodiode.

Differing from a bundle fiber, the multicore plastic optical fiber ofthe present invention can be treated as an optical fiber having onecore. Therefore, it can be fixed at the end of connectors and the likeby caulking coating or adhering with an adhesive agent.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of a multicore plastic optical fiber ofthe present invention.

FIG. 2 is a cross-sectional view of an other multicore plastic opticalfiber of the present invention.

FIG. 3 is a cross-sectional view of a multicore plastic optical fibercable of the present invention.

FIG. 4 is a cross-sectional view of an other multicore plastic opticalfiber cable of the present invention.

The numerals used in the drawings are as follows:

1. core

2. cladding

3. third encapsulating layer

4. coating layer

BEST MODE FOR CARRYING OUT THE INVENTION

The physical properties of the multicore plastic optical fibers used inExamples and Comparative Examples are measured according to thefollowing methods.

(1) Melt Index

Using a melt indexer (manufactured by Toyo Seiki Seisaku-sho, Ltd.),melt index was measured according to ASTM-1238 under the conditions of atest temperature of 230° C., a load of 3.8 kg, an inner diameter of thedie of 2.0955 mm.

(2) Refractive Index

Using an ABBE refractometer (manufactured by ATAGO CO., LTD.),refractive index was measured with the sodium D-line in a room at aconstant temperature of 23° C.

(3) Tensile Break Strength

Tensile break strength was measured according to ASTM D638 at atemperature of 23° C. at a tensile speed of 100 mm/min.

(4) Transmission Loss

Transmission loss was measured according to (i) a 52 m-2 m cutbackmethod and (ii) a 10 m-2 m cutback method.

(i) 52 m-2 m cutback method:

Using monochromatic light having a wavelength of 650 nm as a lightsource, the outgoing optical power P₅₂ of a 52 meter-long optical fiberand the outgoing optical power P₂ of a 2 meter-long optical fiber weremeasured at an incidence opening angle of 0.15 radian, and transmissionloss α₁ per km was obtained according to the following equation:

    α.sub.1 =10000log(P.sub.2 /P.sub.52)/(52-2)           dB/km!

(ii) 10 m-2 m cutback method:

Using monochromatic light having a wavelength of 650 nm as a lightsource, the outgoing optical power P₁₀ of a 10 meter-long optical fiberand the outgoing optical power P₂ of a 2 meter-long optical fiber weremeasured at an incidence opening angle of 0.15 radian, and transmissionloss α₁ per km was obtained according to the following equation:

    α.sub.1 =10000×log(P.sub.2 /P.sub.10)/(10-2)    dB/km!

(5) Bending Characteristics

Using a red semi-conductor laser having a wavelength of 653 nm (653 nmLD), TOLD9421 (S) manufactured by Toshiba Corporation, as light source,the transmitted light capacity of a 3 meter-long optical fiber ismeasured when a middle portion of the fiber is wound round a rod havinga radius of 5 mm to obtain a light capacity value for comparing to thecase when the fiber is not so wound.

(6) Transmission Band

The wavelength at which a transfer function shows -3 dB according to thepulse method is defined as the transmission bandwidth. The transmissionband was measured based on a launch numerical aperture of 0.25, usingoptical fibers being 100 meter long, and 20 meter long, and a 653 nm redsemi-conductor laser (653 nm LD), TOLD 9421 (S) manufactured by ToshibaCorporation as a light source.

EXAMPLES

The present invention is illustrated by reference to the followingExamples, but its scope is not limited by them.

Example 1

As core resin, a methyl methacrylate having a melt flow index of 2 g/10min. and a refractive index of 1.492 was employed. As cladding resin, acopolymer resin, having a melt flow index of 37 g/10 min. and arefractive index of 1.469, and comprising 15 parts by weight ofheprodecafluorodecyl methacrylate, 5 parts by weight of trifluoroethylmethacrylate, 5 parts by weight of tetrafluoropropyl methacrylate and 75parts by weight of methyl methacrylate, was employed. The refractiveindex difference was 0.023.

The core material was melted and fed to a die plate with 19 holesarranged as shown in FIG. 1 using an extruder. The holes of the dieplate were arranged as follows: one at the center, 6 holes around thecenter hole so as to form a regular hexagon, and 12 holes around thehexagon so as to form a regular dodecagon. Then, the cladding resin wasalso fed to the die plate so as to surround the produced core filamentswith the cladding resin, while the core filaments were bundled in atapered die head, so as to bind the cores to obtain a multicore plasticoptical fiber having an almost circular cross-section. The volume ratioof the core resin to the cladding resin was adjusted so as to be 70 to30. The diameter of the resultant multicore plastic optical fiber was1.0 mm, and the average diameter of the cores was 170 μm. Thetransmission loss of the multicore plastic optical fiber was 148 dB/kmwhen measured according to the 52m-2 m cutback method.

The multicore plastic optical fiber was covered with low densitypolyethylene to obtain a multicore plastic optical fiber cable having anouter diameter of 2.2 mm. The transmission loss of the resultantmulticore plastic optical fiber cable was 148 dB/km when measuredaccording to the 52 m-2 m cutback method, which was the same value asthe fiber before coating. The multicore plastic optical fiber cableexhibited satisfactory mechanical properties, i.e., a breaking load of14 kg and breaking extension of 70%.

When the bending characteristics of the multicore plastic optical fibercable were measured, the light capacity retention of the cable onbending was 90% of the cable which was not wound round the rod. When thetransmission bandwidth was measured using a 100 m-long fiber, the cablehad a broad bandwidth of 220 MHz. When the bandwidth was measured usinga 20 m-long fiber, it exceeded the measurement limit of 700 MHz. Theresults are shown in Table 1.

When the multicore plastic optical fiber cable was subjected to a heatresistance test under dry conditions at 85° C., the transmission lossand bandwidth showed no decrease after 2000 hours.

Example 2

A multicore plastic optical fiber was prepared according to the samemethod as in Example 1 except that a die plate with 37 holes wasemployed. The holes of the die plate were arranged as follows: one atthe center, 6 holes around the center hole so as to form a regularhexagon, 12 holes around the hexagon so as to form a regular dodecagon,and 18 holes around the dodecagon so as to form a regular octadecagon.The resultant multicore plastic optical fiber had a diameter of 1.0 mm.The average core diameter was 120 μm. The transmission loss of theresultant multicore plastic optical fiber was 175 dB/km when measuredaccording to the 52 m-2 m cutback method.

The multicore plastic optical fiber was covered with low densitypolyethylene to obtain a multicore plastic optical fiber cable having anouter diameter of 2.2 mm. The transmission loss of the resultantmulticore plastic optical fiber cable was 175 dB/km when measuredaccording to 52 m-2 m cutback method, which was the same value as thefiber before coating.

The bending characteristics and transmission bandwidth of the multicoreplastic optical fiber cable were measured. The results are shown inTable 1.

When the multicore plastic optical fiber cable was subjected to a dryheat resistance test at 85° C., the transmission loss and bandwidthshowed no decrease after 2000 hours.

Example 3

A multicore plastic optical fiber was prepared according to the samemethod as in Example 1 except that a copolymer having a melt flow indexof 39 g/10 min. and a refractive index of 1.482, and comprising 20 partsby weight of butyl acrylate, 5 parts by weight of trifluoromethacrylateand 75 parts by weight of methyl methacrylate, was employed as thecladding resin, and a die plate with 217 holes was employed. The holesof the die plate were arranged as follows: one at the center, 6 holesaround the center core so as to form a regular hexagon, 12 holes aroundthe hexagon so as to form a regular dodecagon, and 217 holes in totalwere arranged in 9 layers, counting the center core as one layer. Theresultant multicore plastic optical fiber had a diameter of 1.0 mm, andthe average core diameter was 52 μm. The transmission loss of the fiberwas 270 dB/km when measured according to the 52 m-2 m cutback method.

The multicore plastic optical fiber was covered with low densitypolyethylene to obtain a multicore plastic optical fiber cable having anouter diameter of 2.2 mm. The transmission loss of the resultantmulticore plastic optical fiber cable was 270 dB/km when measuredaccording to the 52 m-2 m cutback method, which was the same value asthe fiber before coating.

The bending characteristics and transmission bandwidth of the multicoreplastic optical fiber were measured. The results are shown in Table 1.

Example 4

Using a die plate with 37 holes as employed in Example 3 and a copolymerresin having a melt flow index of 38 g/10 min. and a refractive index of1.485, and comprising 22.5 parts by weight of butylacrylate and 77.5parts by weight of methyl methacrylate as the cladding resin, amulticore plastic optical fiber having an almost circular cross-sectionwas prepared. The cladding resin was fed to the die plate so that thevolume ratio of the core resin to the cladding resin might be 60 to 40.The resultant multicore plastic optical fiber had a diameter of 0.5 mm,and the average core diameter was 60 μm. The transmission loss of thefiber was 198 dB/km when measured according to the 52 m-2 m cutbackmethod.

The multicore plastic optical fiber was covered with low densitypolyethylene to obtain a multicore plastic optical fiber cable having anouter diameter of 2.2 mm. The transmission loss of the resultantmulticore plastic optical fiber cable was 198 dB/km when measuredaccording to the 52 m-2 m cutback method, which was the same value asthe fiber before coating.

The bending characteristics and transmission bandwidth of the multicoreplastic optical fiber were measured. The results are shown in Table 1.

Example 5

A multicore plastic optical fiber having 19 cores was prepared accordingto the same method as in Example 1 except that a copolymer having a meltflow index of 36 g/10 min. and a refractive index of 1.454, andcomprising 24 parts by weight of heptadecafluorodecyl methacrylate, 8parts by weight of trifluoromethacrylate, 8 parts by weight oftetrafluoropropyl methacrylate and 60 parts by weight of methylmethacrylate, was employed as the cladding resin. The resultantmulticore plastic optical fiber had a diameter of 1.0 mm and an averagecore diameter was 170 μm. The transmission loss of the fiber was 150dB/km when measured according to the 52 m-2 m cutback method.

The multicore plastic optical fiber was covered with low densitypolyethylene to obtain a multicore plastic optical fiber cable having anouter diameter of 2.2 mm. The transmission loss of the resultantmulticore plastic optical fiber cable was 150 dB/km when measuredaccording to the 52 m-2 m cutback method, which was the same value asthe fiber before coating.

The bending characteristics and transmission bandwidth of the multicoreplastic optical fiber were measured. The results are shown in Table 1.

Examples 6 and 7

A plastic optical fiber was prepared according to the same method as inExample 1 except that the conditions were changed as shown in Table 1,and its properties were measured. The results are shown in Table 1.

Comparative Example 1

As core resin, a methyl methacrylate having a melt flow index of 2 g/10min. and a refractive index of 1.492 was employed. As cladding resin, acopolymer having a melt flow index of 37 g/10 min. and a refractiveindex of 1.410, and comprising 70 parts by weight ofheptadecafluorodecyl methacrylate, 12 parts by weight of trifluoroethylmethacrylate, 12 parts by weight of tetrafluoropropyl methacrylate and 6parts by weight of methyl methacrylate, was employed. The refractiveindex difference was 0.082.

A single-core plastic optical fiber comprising a core resin of a methylmethacrylate resin and a cladding resin of the above copolymer resin,and having an outer diameter of 1.0 mm and a core diameter of 980 μm,was prepared. The resultant plastic optical fiber was covered with lowdensity polyethylene to obtain an plastic optical fiber cable having anouter diameter of 2.2 mm.

The transmission loss and bending characteristics of the fiber cablewere measured. The results are shown in Table 1. The transmissionbandwidth was relatively narrow.

Comparative Example 2

Using the core resin and the cladding resin employed in Example 1, asingle-core plastic optical fiber having an outer diameter of 1.0 mm anda core diameter of 980 μm was spun. The resultant plastic optical fiberwas covered with low density polyethylene to obtain a single-coreplastic optical fiber having an outer diameter of 2.2 mm.

The transmission loss, bending characteristics and transmissionbandwidth were measured. The results are shown in Table 1. The bendingcharacteristics were unsatisfactory.

Comparative Example 3

Using the core resin and the cladding resin employed in Example 1, asingle-core plastic optical fiber having an outer diameter of 0.25 mmand a core diameter of 240 μm was prepared. The resultant plasticoptical fiber was covered with low density polyethylene to obtain asingle-core plastic optical fiber having an outer diameter of 1.0 mm.

The transmission loss, bending characteristics and transmissionbandwidth were measured. The results are shown in Table 1. The bendingcharacteristics was not sufficient.

Comparative Example 4

A multicore plastic optical fiber with 217 cores was prepared accordingto the same method as in Example 3 except that a copolymer resin havinga melt flow index of 38 g/10 min. and a refractive index of 1.489, andcomprising 10 parts by weight of butyl acrylate and 90 parts by weightof methyl methacrylate, was employed as the cladding resin. Theresultant multicore plastic optical fiber had a diameter of 1.0 mm andan average core diameter was 52 μm. The transmission loss of themulticore plastic optical fiber was 400 dB/km when measured according tothe 10 m-2 m cutback method.

Further, the multicore plastic optical fiber was covered with lowdensity polyethylene to obtain a multicore plastic optical fiber cablehaving an outer diameter of 2.2 mm. The transmission loss of the cablewas 400 dB/km when measured according to the 10 m-2 m cutback method,which was the same value as the fiber before coating.

The bending characteristics and transmission bandwidth of the multicoreplastic optical fiber cable were measured. The results are shown inTable 1.

Comparative Example 5

A multicore plastic optical fiber with 19 cores was prepared accordingto the same method as in Example 1 except that a copolymer resin havinga melt flow index of 36 g/10 min. and a refractive index of 1.444, andcomprising 30 parts by weight of heptadecafluorodecyl methacrylate, 10parts by weight of trifluoroethyl methacrylate, 10 parts by weight oftetrafluoropropyl methacrylate and 50 parts by weight of methylmethacrylate, was employed as the cladding resin. The resultantmulticore plastic optical fiber had a diameter of 1.0 mm and the averagecore diameter was 170 μm. The transmission loss of the multicore plasticoptical fiber was 150 dB/km when measured according to the 52 m-2 mcutback method.

Further, the multicore plastic optical fiber was covered with lowdensity polyethylene to obtain a multicore plastic optical fiber cablehaving an outer diameter of 2.2 mm. The transmission loss of the cablewas 150 dB/km when measured according to the 52 m-2 m cutback method,which was the same value as the fiber before coating.

The bending characteristics and transmission bandwidth of the multicoreplastic optical fiber cable were measured. The results are shown inTable 1.

Comparative Example 6

A multicore plastic optical fiber was prepared according to the samemethod as in Example 1 except that a die plate with 3500 holes wasemployed. The 3500 holes of the die plate employed were arranged so thatany three adjacent holes formed a regular triangle and the outermostholes defined a substantially circular shape, to achieve a closestpacking structure.

The resultant multicore plastic optical fiber had a diameter of 3.0 mmand the average core diameter was 43 μm. The transmission loss of themulticore plastic optical fiber was 420 dB/km when measured according tothe 10 m-2m cutback method.

The bending characteristics and transmission bandwidth of the multicoreplastic optical fiber were measured. The results are shown in Table 1.

Comparative Example 7

A plastic optical fiber was prepared according to the same method as inComparative Example 6 except that the conditions of Comparative Example6 were changed as shown in Table 1, and its properties were measured.The results are shown in Table 1.

Industrial Applicability

The multicore plastic optical fiber for light signal transmission of thepresent invention has broader transmission bandwidth and a lowertransmission loss and exhibits more excellent bending characteristicscompared to conventional plastic optical fibers. The multicore plasticoptical fiber of the present invention can transmit light signals inshort and medium distances at a high speed. Accordingly, the multicoreplastic optical fiber of the present invention is suitable forinformation transmission such as LAN, FA, OF and computer networks

                                      TABLE 1                                     __________________________________________________________________________                                          Transmission                                                                  bandwidth                               Outer                        Tans-                                                                             Bending                                                                            100 m                                                                              20 m                               diameter   Core     Refractive Index n                                                                     mission                                                                           character-                                                                         measure-                                                                           measure-                           of fiber   diameter                                                                           Core   Clad- loss                                                                              istics                                                                             ment ment                               μm      μm                                                                              number                                                                            Core                                                                             ding                                                                             Δn                                                                         dB/km                                                                             %    MHz  MHz                                __________________________________________________________________________    Example                                                                       1     1000 170  19  1.492                                                                            1.469                                                                            0.023                                                                            148 90   220  >700                               2     1000 120  37  1.492                                                                            1.469                                                                            0.023                                                                            175 96   220  >700                               3     1000  52  217 1.492                                                                            1.482                                                                            0.010                                                                            270 90   400  >700                               4      500  60  37  1.492                                                                            1.485                                                                            0.007                                                                            198 85   500  >700                               5     1000 170  19  1.492                                                                            1.454                                                                            0.038                                                                            150 95   130   650                               6      670 114  19  1.492                                                                            1.469                                                                            0.023                                                                            147 96   220  >700                               7      500  85  19  1.492                                                                            1.469                                                                            0.023                                                                            173 96   210  >700                               Comparative                                                                   Example                                                                       1     1000 980   1  1.492                                                                            1.410                                                                            0.082                                                                            140 83    85   400                               2     1000 980   1  1.492                                                                            1.469                                                                            0.023                                                                            145 25   210  >700                               3      250 240   1  1.492                                                                            1.469                                                                            0.023                                                                            200 70   210  >700                               4     1000  52  217 1.492                                                                            1.489                                                                            0.003                                                                            400 60        >700                               5     1000 170  19  1.492                                                                            1.444                                                                            0.048                                                                            150 95   100   500                               6     3000  43  3500                                                                              1.492                                                                            1.469                                                                            0.023                                                                            420 >96       >700                               7     1000  15  3500                                                                              1.492                                                                            1.469                                                                            0.023                                                                            480 >96       >700                               __________________________________________________________________________

We claim:
 1. A multicore plastic optical fiber for light signaltransmission comprising 7 or more cores having a diameter of 50 to 200μm which are covered with a cladding resin having a refractive indexlower than that of the core resin by 0.005 to 0.04.
 2. The multicoreplastic optical fiber for light signal transmission according to claim1, wherein the transmission bandwidth is 625 MHz·20 m or more whenmeasured using a light source having a launch numerical aperture of0.25.
 3. The multicore plastic optical fiber for light signaltransmission according to claim 1, wherein the transmission bandwidth is125 MHz·100 m or more when measured using a light source having a launchnumerical aperture of 0.25.
 4. The multicore plastic optical fiber forlight signal transmission according to claim 1, wherein the core resinis a methyl methacrylate type resin.